1. Field of the Invention
The present invention relates to an electronic endoscope apparatus having a video-scope with an image sensor, especially, it relates to the recording of a still-image.
2. Description of the Related Art
In an electronic endoscope apparatus having a video-scope with an image sensor, a subject image, such as a diseased portion, is displayed on a monitor in real-time. Namely, a moving-image is displayed. Further, the electronic endoscope apparatus is capable of displaying a still-image on a monitor and recoding the still-image in an image memory. When an operator pushes a freezing switch button provided on an operating portion of the video-scope, a series of actions (so called “freeze action”) is performed. Consequently, the still-image is displayed, and can be recorded, or printed.
When displaying or recording the observed portion in an organ as a still-image, the degradation of resolution or the degradation of color sharpness occurs due to a hand tremble of the video-scope, or movement of the observed portion. Consequently, the image-quality degrades. Especially, the image-quality significantly degrades when the distance between the observed portion and the distal end of the video-scope is far, or when enlarging the displayed subject image by a zooming function. To solve the above problems, it is known that, when displaying/recording the still-image, the amount of light for illuminating the subject should be increased and the electronic shutter speed of the image sensor should be set to a higher speed. In this case, the light-amount is increased by fully opening a diaphragm or by emitting a strobe light, and the electronic shutter speed is set in accordance with the increase of light-amount.
However, since the increased light-amount is predetermined regardless of quality of the still-image, the proper electronic shutter speed should be set while repeatedly recoding the still image and repeatedly judging whether the brightness of the recorded still-image is proper. Further, when the opening-degree of the diaphragm immediately before the freeze action is very small, the light-amount rapidly increases due to fully opening the diaphragm, almost instantaneously, so that a still-image with a proper brightness cannot be obtained.